Radiation Image Read-Out and Cropping System

ABSTRACT

A system comprising a read out device for reading a radiation image stored in a photostimulable phosphor screen and for generating a digital signal representation of a read out image wherein the read out device supports a single format and is coupled to a processing unit which is programmed for cropping a region of interest (ROI) in the read out image.

BACKGROUND OF THE INVENTION

In computed radiography it is nowadays common practice to record aradiation image on a photostimulable phosphor screen by exposing thescreen to an image-wise pattern of penetrating radiation such as X-rays.

The latent image stored in the photostimulable phosphor screen is readout by stimulating the phosphor with light having (a) wavelength(s)within the stimulation wavelength range of the phosphor.

The light emitted upon stimulation is then detected and converted intoan electronic signal representation of the image.

The electronic signal can then be processed and can be archived,displayed or printed.

Several types of devices for reading radiation images stored inphotostimulable phosphor screens, adapted to the needs of hospitals orradiologists are nowadays marketed. Most of these devices are adaptedfor read out of different formats of photostimulable phosphor screens.Different formats are provided for different kinds of examinations.

However, the fact that a device is adapted to support different formatsmakes the device expensive. Furthermore the use of different formatsdemands an extra decision to be made by the operator so as to select theadequate format for a type of examination.

So devices have been developed that are less complex and can only readout a single format.

This implies that the user can only use a single format photostimulablephosphor screen for different kinds of examinations.

If this single format is large compared to the body part that is to beexamined, e.g. in case of a radiation image of a small body part such asa hand or a finger, the area outside the body part is commonly coveredby x-ray opaque material. The area on the phosphor screen which iseffectively used is then rather limited.

SUMMARY OF THE INVENTION

When such a large format photostimulable phosphor screen is read out,this gives rise to a large data file containing a lot of data which arenot relevant for examination because they belong to the shielded areaoutside of the body part. Large data files occupy a large amount ofprocessing and or storage capacity.

Moreover, the region outside the small region of diagnostic relevancewill have high luminance when a hard copy image is generated anddisplayed on a display screen. The strong light will have a negativeimpact on the efficiency and accuracy of the diagnosis.

In case of multiple exposures, more than one x-ray image is recorded ondifferent parts of the same photostimulable phosphor screen during morethan one successive exposure steps. For example when images of right andleft hand are to be examined, these images are often generated onjuxtaposed parts of the same phosphor screen, one part of the screenbeing exposed while the other part is covered with x-ray shieldingmaterial. When the stimulable photostimulable phosphor screen is thenread out, the digital image signal is composed of the images of the twobody parts, as well as of the area surrounding each of the images.

When the exposed objects are small (hands, fingers) the entire read outimage comprises a large area which is unimportant for diagnosis.

So, also in case of multiple exposures the above-described problems areencountered.

The present invention relates to a system for reading a radiation imagethat has been stored in a photostimulable phosphor screen.

It is an object of the present invention to provide a system thatovercomes the above-mentioned disadvantages of the prior art.

In general, according to one aspect, the invention features a systemcomprising a device for reading a radiation image stored in aphotostimulable phosphor screen and for generating a digital signalrepresentation of a read out image. The device is adapted to read asingle format screen. A processing unit is coupled to the reading deviceand is programmed for cropping a region of interest (ROI) in said image.

In embodiments, the image is obtained by multiple exposure of saidsingle format photostimulable phosphor screen.

In general, according to another aspect, the invention features a methodfor processing an image. The method comprises reading a radiation imagestored in a photostimulable phosphor screen, generating a digital signalrepresentation of a read out image with a device that is adapted to reada single format screen, and cropping a region of interest (ROI) in saidimage.

The above and other features of the invention including various noveldetails of construction and combinations of parts, and other advantages,will now be more particularly described with reference to theaccompanying drawings, and pointed out in the claims. It will heunderstood that the particular method and device embodying the inventionare shown by way of illustration and not as a limitation of theinvention. The principles and features of this invention may be employedin various and numerous embodiments without departing from the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, reference characters refer to the sameparts throughout the different views. The drawings are not necessarilyto scale; emphasis has instead been placed upon illustrating theprinciples of the invention. Of the drawings:

FIG. 1 shows the components of a device for reading a radiation imagethat has been stored in a photostimulable phosphor screen,

FIG. 2 shows a commercially available apparatus for reading out an imagethat has been stored in a single-format photostimulable phosphor screen,

FIG. 3 is an image originating from multiple exposures of aphotostimulable phosphor screen,

FIG. 4 is an image obtained after applying auto-cropping.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A table top size read out device (Agfa CR 10-X) that is adapted to reada single format photostimulable phosphor screen is shown in FIG. 2. Thisdevice has a feeding opening for receiving photostimulable phosphorscreens of a single, specific size. The commercial device that is shownis adapted to read out a screen of 35 centimeters (cm)×43 cm at aresolution of 10 pixels/millimeters (mm) (these data are only exemplaryand not meant to be limitative for the present invention).

In one embodiment a device of the described kind can be a mobile devicethat can be transported to the location where a radiographic image isgenerated and read out. Such a mobile device is e.g. very useful inveterinary applications.

FIG. 1 shows a specific embodiment of a read out device which is adaptedfor read out of a radiation image that has been stored in aphotostimulable phosphor screen.

The apparatus comprises a stimulating light source 2 such as a laser anda galvanometric mirror 4, driven by drive unit 5 arranged to deflect thelight emitted by the light source in the fast scan direction.

The apparatus further comprises transport unit (not shown) to transportthe screen in the direction of arrow 6 to enable the screen to betwo-dimensionally scanned.

Positioned close to but behind the scanning line of the laser beam onthe phosphor sheet 1 is a light guide 7 which receives image-wisemodulated light emitted from the phosphor sheet but is shielded byshielding unit 8 from direct exposure to the laser beam 3. The lightguide comprises individual optical fibers that are bundled at the outputend. The output end is positioned close to the entrance of a lightdistributing means 9 such as a hollow tube-like member with a reflectinginner surface.

The output end of the tube is positioned adjacent to the entrance windowof a photo multiplier 10 which produces an electrical signalrepresentative of the light intensity falling on its entrance window. Anoptical filter (not shown) was placed at the output window of the tubein front of the photomultiplier.

Light originating from a single pixel in the image is transported by atleast one of the optical fibers to entrance of the tube and is thenfurther guided by the tube onto the entrance window of thephoto-multiplier where the light falling on the entire surface will beintegrated and will constitute the input of the light to signalconversion for the generation of the signal presentation of thatspecific pixel.

The signal generated by the photo multiplier is processed in imageprocessing unit 11.

It is also transmitted to a further processing unit (not shown), alsocalled workstation, coupled to the read out device where it can besubjected to additional processing and can be displayed and/or stored.

The processing unit 11 according to this invention is programmed toperform a cropping operation on the image read out of a phosphor screenwithout interference of an operator (auto-cropping functionality).

Cropping operates on a read out image to create a new image by selectinga desired portion (region of interest) from the image. Next the croppedregion of interest is made to cover the entire image area, in aso-called it to view lay-out, while the parts outside the region ofinterest are discarded.

An example of a method for automatically detecting a collimated area inan image has been described in European patent application 2 500 864.

When applying the described algorithm a polygon is obtained whichdelineates the region of interest from the collimated border area (areathat was shielded from irradiation).

By means of this polygon a so-called bounding box is calculated thatincludes this polygon. The bounding box determines the area that will befit to view.

The described method is a region-based method of recognizing anirradiation field in digital x-ray image. Region-based refers to thefact that candidate irradiation field boundaries are computed out of asegmented map of the image and not directly out of the greyscale imageobtained by read out of the irradiated phosphor screen.

The method described in EP 2 500 864 A is a 3-step process. Thisapplication relates to PCT/EP2012/03252, filed on 27 Feb. 2013, whichclaims priority to U.S. Provisional Application No. 61/452,810, filed onMar. 15, 2011, both of which are incorporated herein by reference intheir entirety.

In the first step the image is segmented in multiple regions eachcomprising pixels which have similar local image characteristics, e.g.by applying a multi-scale watershed method.

The second step is a step of fitting line segments to the regionboundaries whereby the line segments are candidate irradiation fieldboundaries and constitute a segmentation map.

The third step is identifying in the new segmentation map the regionscorresponding to irradiation fields using local and/or regional and/orglobal image characteristics. This is performed by classifying regionsin the segmentation map into at least two classes, one class beingirradiation field and the other class being collimated region on thebasis of at least one of local, regional and global imagecharacteristics.

The segmentation of the image in multiple regions may be improved usingimage clustering to merge regions which have similar local imagecharacteristics.

In one embodiment the clustering technique is hierarchical clusteringwith the measure of similarity based on at least one of the median oraverage greyscale pixel value of a segmented region, the standarddeviation of the greyscale pixel values within a segmented region andthe position of the segmented region in the image.

A Hough transform may be applied to the boundaries of the segmentedregions to fit line segments corresponding with candidate irradiationfield boundaries.

The applied Hough transform may be normalized and corrected in a waythat the Hough space values of the boundaries of the irradiation fieldsin the image approximate value 1.0.

In one embodiment only line segments are preserved that have asignificant overlap with the boundaries of the clustered regions, arepreserved.

Regional characteristics are e.g. computed out of the histograms oflocal standard deviation of the different segmented regions, for examplea distance measurement between said histograms and a histogram of localstandard deviation of the total image.

Another example of such a characteristic is a distance measurementbetween said lust grants and a reference histogram of local standarddeviation of only the brightest regions in the image.

Still another example is the cumulative sum of the histogram below aspecified histogram abscissa.

Examples of regional characteristics are: the amount of strong edges inthe different segment regions, the average greyscale pixel differencebetween a region of interest and its surroundings in the neighbourhoodof the boundaries of the said region of interest.

In one embodiment the binary classification is performed by using aperceptron.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. A system comprising a device for reading aradiation image stored in a photostimulable phosphor screen and forgenerating a digital signal representation of a read out image, saiddevice being adapted to read a single format screen, a processing unitcoupled to said reading device, said processing unit being programmedfor cropping a region of interest (ROI) in said image.
 2. A systemaccording to claim 1 wherein said image is obtained by multiple exposureof said single format photostimulable phosphor screen.
 3. A method forprocessing an image, comprising reading a radiation image stored in aphotostimulable phosphor screen; generating a digital signalrepresentation of a read out image with a device that is adapted to reada single format screen, and cropping a region of interest (ROI) in saidimage.
 4. A method according to claim 3 further comprising obtainingsaid image by multiple exposure of a single format photostimulablephosphor screen.